FIG. 8 shows a ducted fan gas turbine engine 10 comprising in axial flow series: an air intake 12, a propulsive fan 14 having a plurality of fan blades 16, an intermediate pressure compressor 18, a high-pressure compressor 20, a combustor 22, a high-pressure turbine 24, an intermediate pressure turbine 26, a low-pressure turbine 28 and a core exhaust nozzle 30. A nacelle (not shown) generally surrounds the fan casing 32 and engine 10 and defines the intake 12, a bypass duct 34 and a bypass exhaust nozzle. The engine has a principal axis of rotation 31.
Air entering the intake 12 is accelerated by the fan 14 to produce a bypass flow and a core flow. The bypass flow travels down the bypass duct 34 and exits the bypass exhaust nozzle 36 to provide the majority of the propulsive thrust produced by the engine 10. The core flow enters in axial flow series the intermediate pressure compressor 18, high pressure compressor 20 and the combustor 22, where fuel is added to the compressed air and the mixture burnt. The hot combustion products expand through and drive the high, intermediate and low-pressure turbines 24, 26, 28 before being exhausted through the nozzle 30 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines 24, 26, 28 respectively drive the high and intermediate pressure compressors 20, 18 and the fan 14 by concentric interconnecting shafts 38, 40, 42.
As would be expected, bearings are employed throughout the engine to allow various components to rotate. These bearings are designed and made to be able to withstand the operating conditions exerted on them during use. One type of bearing which finds common use is a cylindrical roller bearing which can be used to provide rotational support to the interconnecting shafts.
Cylindrical roller bearing raceways are manufactured with a cylindrical raceway (round and perpendicular to a datum surface) to within a tight tolerance. Typically, for aerospace standard bearings the raceway will have a perpendicularity of the order of 0.005 mm for a raceway diameter of around 200 mm. This geometric control is to ensure a suitable line contact can be achieved between the raceway and the roller. If the raceway is not perpendicular to the roller this can lead to a skewed contact profile which can cause increased contact stress, dynamic instability, cage pocket wear and bearing failure.
To accommodate misalignment, rollers typically have a profile comprising a central flat length, a crowned section (of large radius) to both sides of the flat length, and a small corner radius at both ends of the roller. This profile allows the roller bearing to accommodate misalignment by minimising the increase in contact stress at the edge of the contact patch through gradual relief of the contact across the crowned section of the roller.
Misalignment of a roller bearing can be caused through manufactured geometric tolerances, eccentricity of installation in to a rotor system, and operational conditions.
The operational environment of a bearing may exceed reliable misalignment levels through a combination of rotor geometry, weight distribution, structural deformation, and temperature gradients. If it is not possible to ensure the bearing is operating within reliable levels of misalignment by addressing any of these issues then the bearing is likely to become distressed.
The invention seeks to address these issues by providing a bearing which is able to cope with greater misalignment caused by operational conditions.